The $14 billion project, of which $4 billion was already spent on steps like digging a foundation and laying water pipes, will be closely watched by utilities around the country, many of which are leery of nuclear construction because of huge cost overruns in the last round of construction in the 1960s, ’70s and ’80s.

At the same time, many are intent on seeking an alternative to natural gas, the fossil fuel now dominating construction plans.

The Southern Company is adding the two reactors to its Alvin W. Vogtle nuclear plant near Augusta, in tandem with several partners that serve municipal utilities or electric cooperatives.

“It is a landmark, landmark achievement,” said Paul Bowers, president of Georgia Power, the Southern subsidiary that is building the plant. The company applied for the license four years ago under a new system meant to reduce the expense and uncertainty of construction. Still, only one other project, a twin-unit plant in South Carolina, seems likely to be built soon.

The sole vote against approval was cast by the commission’s chairman, Gregory B. Jaczko. He said the license would not assure that all of the safety improvements sought by the agency in response to Japan’s Fukushima disaster would be accomplished before the reactors begin operating in 2016 and 2017.

“I cannot support issuing this license as if Fukushima had never happened,” said Mr. Jaczko, who is frequently at odds with his fellow commissioners.

But his colleagues said they were confident that the agency’s regular rule-making process would ensure that all appropriate safety changes are carried out and that public health and safety are protected.

The design for the two new reactors is a new one: the Westinghouse AP1000, which is intended to withstand earthquakes and plane crashes and to be less vulnerable to a cutoff of electricity, which set off the triple meltdown at Fukushima in 2011.

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An emergency water tank is perched above the reactor so that no pumps are needed to deliver cooling water, and the design has far fewer valves than most reactors now in service. Whereas reactors typically have a heavy concrete dome with a steel liner, the AP1000 allows air to circulate between the concrete shield building and the liner, so that natural convection will remove excess heat in an emergency.

Thomas A. Fanning, Southern’s chairman, said in a conference call with reporters that the company would “incorporate the comments of everybody and every event along the way, to assure that we have the safest, most reliable generation in the world built here at Plant Vogtle.”

But he acknowledged that the changes sought by the commission, not all of which have been established, might not be accomplished before commercial operation begins.

The reactors will join two units that have operated since the late 1980s at Vogtle, which will become the largest nuclear power complex in the country. If all goes well, some 5,000 employees could be working at the site in two years, up from 1,500 now.

A coalition of antinuclear groups said it would file suit to block the commission’s decision because the lessons of Fukushima had not been applied to the design. Business groups including the United States Chamber of Commerce and the American Public Power Association offered fervent praise.

Senator Lamar Alexander, a Tennessee Republican who is a vocal supporter of nuclear power in Congress, urged the commission to “act promptly on the applications awaiting approval for 14 reactors of the same model.”

But aside from South Carolina Electric and Gas, most companies are waiting to see how construction goes at Vogtle, how the current low price for natural gas moves and how many coal plants must shut down to comply with new federal emissions rules.

A version of this article appears in print on February 10, 2012, on Page B3 of the New York edition with the headline: Federal Regulators Approve Two Nuclear Reactors in Georgia. Order Reprints|Today's Paper|Subscribe